I think Ardweeny is smaller!
I think this is smaller, but its not something you can buy (this link was posted in another thread):
http://www.geocities.jp/arduino_diecimila/obaka/project-2/index_en.html
[edit]I must note that the above isn't strictly an "Arduino" - it uses an ATMega88 @ 8Mhz, leaving out the crystal/resonator, etc...[/edit]